In engines that perform lean combustion, such as diesel engines, an operating range in which a mixture having a high air-fuel ratio (lean atmosphere) accounts for most of the entire operation range, causing concern regarding an emission of relatively large amounts of nitrogen oxides (hereinafter referred to as NOx). In order to address this problem, a NOx storage catalyst for storing (absorb) NOx contained in exhaust gas is installed in the exhaust path of an engine. NOx is stored in this NOx storage catalyst to clean the exhaust gas.
A known configuration for suppressing the amount of NOx generated by combustion in a combustion chamber is an exhaust gas recirculation (EGR) apparatus that recirculates part of exhaust gas in an intake path (for example, see PTL 1 and PTL 2 below).
The aforementioned EGR apparatus is provided with an EGR path by which an engine exhaust path and intake path are in communication with each other, and an EGR valve provided in the EGR path. By adjusting the opening degree of the EGR valve, the amount (EGR amount) of exhaust gas that is recirculated from the exhaust path to the intake path via the EGR path is adjusted to set an EGR rate in the intake air to a target EGR rate that has been set in advance. When part of the exhaust gas is returned to the intake path by the EGR apparatus in this way, the combustion temperature in the combustion chamber is reduced so that NOx generation is suppressed, and as a result, exhaust emissions are improved.
On the other hand, when incomplete combustion of an air-fuel mixture occurs in the combustion chamber in the expansion (combustion) stroke of the aforementioned diesel engine, smoke is generated in the exhaust gas, resulting in deteriorated exhaust emissions. To reduce the amount of this smoke, a technique has been proposed in which the main injection, which is fuel injection for obtaining engine torque, is divided into a plurality of divided main injections. In this case, the injection amount per main injection is reduced to overcome the shortage of oxygen in a combustion field, thus suppressing smoke generation.